1. Field of the Invention
This invention relates to a 1-phase energized disk-type brushless motor.
2. Description of the Prior Art
A 1-phase or single phase motor has a dead point at an energization switching point at which the motor provides zero torque. Accordingly, the 1-phase motor has the drawback that it cannot start itself if the rotor position upon starting of the motor is just at a dead point.
Therefore, a 1-phase motor is normally provided with a cogging generating magnetic member (an iron piece is used therefor) for generating a torque (cogging torque) in addition to a torque generated by an armature coil and a field magnet (rotor magnet) in order to eliminate such dead points, to allow self-starting of the motor.
In a coreless motor, for example, the following methods for generating a cogging torque are known. Referring first to FIG. 1, a 6-pole field magnet or magnet rotor 2 having an alternate arrangement of the 6 north and south poles is mounted on a rotor yoke 1 in an opposing relationship to a stator yoke 5 with an air gap 4 left therebetween and with a pair of coreless armature coils 3 disposed in the air gap 4. In the motor of FIG. 1, the stator yoke 5 has at a face thereof opposing the field magnet 2 two inclined surfaces which thus define the complementarily inclined air gap 4. This method, however, has a drawback that the efficiency is relatively low because the air gap is relatively great.
Referring now to FIG. 2, another method is illustrated. In the motor of FIG. 2, a stator yoke 5 has no such inclined faces as provided on the stator yoke 5 of FIG. 1. Instead, an iron bar 6 is mounted on the stator yoke 5 and extends through each of a pair of coreless armature coils 3 disposed in a uniform air gap 4 defined by the stator yoke 5 and a field magnet or magnet rotor 2 on a rotor yoke 1. According to this arrangement, a magnetic flux will appear as seen in FIG. 3 and hence the field magnet 2 will stop at a position in which the iron bars 6 are each opposed to the center of one of the N and S poles of the field magnet 2. Accordingly, if the armature coils 3 are located so as to produce a rotational torque in such a stopping position of the field magnet 2, a coreless motor which can start itself will be obtained.
However, the method as shown in FIG. 2 has a drawback that if the thickness of the iron bars 6 is increased in order to increase the cogging torque, a phenomenon that the torque around dead points decreases will appear because a magnetic flux 7 will act as shown in FIG. 4 around the dead points.
In order to obtain an ideal torque-angular rotor displacement curve, it is necessary to obtain a composite torque curve 8 as shown in FIG. 5. In FIG. 5, an armature coil torque curve of an armature coil is indicated by a curve 9 while a cogging torque curve of a cogging generating magnetic member is indicated by a curve 10. As apparent from the armature coil torque curve 9 and the cogging torque curve 10, the cogging torque should be a half of the armature torque in magnitude. Accordingly, the torque curve 8 composite of the armature coil torque and the cogging torque exhibits a substantially uniform rotational torque over the entire range of rotation.
In order to obtain such an ideal composite torque curve 8, a cogging magnetic member must be designed correctly in size and location.